A piezoelectric fuel injector comprises a piezoelectric actuator, which converts an electrical drive signal into a mechanical lifting movement. A nozzle needle is controlled by said lifting movement, with which the fuel flow through the injection holes of a nozzle unit can be more or less enabled in order to enable a desired quantity of fuel to be injected into a cylinder of the vehicle in a suitable manner depending on the electrical drive signal.
A piezoelectric actuator has the property that it outputs an electric signal in the event of a mechanical pressure load, so that it can also be used as a sensor for detection of the prevailing pressure in the piezoelectric injector. By means of said type of detection of the current pressure or a pressure change and the resulting force actions on the piezoelectric element, conclusions can be drawn regarding the lifting movement of the nozzle needle.
Furthermore, the operation of a piezoelectric injector having a piezoelectric actuator and a nozzle needle movable thereby at different times in a partial stroke movement and in a full stroke movement is already known. The piezoelectric actuator is thereby acted upon by a charge amount from a voltage, current and charge source, which has a voltage value dependent on the current injection profile demand and extends the piezoelectric actuator in order to move the nozzle needle in a respectively required manner. In partial stroke mode only part of the possible flow cross-section is opened, so that the flow is choked and only a relatively small quantity of fuel is injected into the respective cylinder, and the nozzle needle is not moved as far as its mechanical stop position. In full stroke mode the maximum possible flow cross-section is fully opened, so that the choke effect is completely removed and a relatively large amount of fuel is injected into the respective cylinder, and the nozzle needle is in its mechanical stop position.
A method and a device for shaping an electrical control signal for an injection pulse are known from WO 2009/010374 A1. Using the curve profile of the electrical control signal, the injection rate of the fuel injector is controlled depending on the rail pressure, on the stroke travel and/or on the opening duration of the fuel injector. The profile of the electrical control signal can be formed freely in relation to at least one pulse edge and/or amplitude for at least one partial quantity to be injected. Said shaping of the injection pulse is carried out in such a way that the specified amount of fuel to be injected is kept constant independently of the profile of the electrical control signal. By means of the injected partial quantity, the injected fuel quantity achieves an intermediate level, which is maintained for a specified holding time.
Other known injection systems for internal combustion engines, especially those with a piezoelectric drive, use methods for improvement of the injection behavior, whereby reference measurements are carried out in the factory during the manufacture or the final testing of the injectors and correction values obtained using said reference measurements are provided for the respective injector depending on the specimen. Such methods are e.g. known from DE 102 15 610 A1 and DE 10 2004 053 266 A1.
During installation of the injector in an internal combustion engine, said correction values are transmitted to the injection controller. From this, especially in relation to current emission legislation requirements, it is necessary to show a definite relationship between a respective injector and respective associated correction values. Furthermore, producing a matrix of correction values and the transfer of the values into the controller requires carrying out multiple working steps, which is associated with a non-negligible outlay of time.
The injection behavior of an internal combustion engine is in principle to be adjusted so that applicable legal regulations in relation to exhaust emissions and fuel consumption are satisfied. Compliance with said regulations is currently guaranteed using other sensors, including e.g. cylinder pressure sensors and/or knock sensors.
In DE 10 2010 040 253.2 a method for monitoring the state of a piezoelectric injector of a fuel injection system having a piezoelectric actuator and a nozzle needle movable thereby is described, with which the piezoelectric injector can be operated in a partial stroke mode and in a full stroke mode. This method involves the detection of electrical measurement values of the piezoelectric injector in partial stroke mode, a comparison of the recorded electrical measurement values with associated comparison values and the drawing of conclusions regarding the state of the piezoelectric injector from the comparison result.